You are welcome to book a lesson that is split/shared between siblings, a parent/child or friends.
The price would be the same as if it were just one person, but an extra £3 per lesson. A one hour lesson is normally £45, so a split/shared 60 minute lesson would be £48 total.
This is because split/shared lessons will always involve more preparation and active teaching time than a more relaxed, slower-paced lesson with one pupil. But it still gives you a discount on the price for two. A 45 minute lesson shared would be £38.
Note that a shared lesson (teaching two at the same time) must be two people of the same standard who are happy to learn the same songs. Sharing lessons in this way is often not as efficient as it sounds. If one person is making progress faster than the other, it can mean much slower progress for one, and extremely discouraging for the other. It can often be better in the long term to split the lesson, and be taught individually.